Heat transfer loops are often placed in the earth to provide for the heating and cooling of residential and commercial spaces. Since ground temperatures are generally similar to room temperatures in buildings, the use of such heat transfer loops can be cost effective alternatives to conventional heating and cooling systems. The installation of such heat transfer loops involves inserting a continuous loop of pipe connected to a heat pump unit into a hole or series of holes in the earth to act as a heat exchanger. A thermally conductive grout is then placed in the hole between the pipe wall and the earth. A heat transfer fluid can be circulated through the underground heat transfer loop to allow heat to be transferred between the earth and the fluid via conduction through the grout and the pipe wall. When the system is operating in a heating mode, a relatively cool heat transfer fluid is circulated through the heat transfer loop to allow heat to be transferred from the warmer earth into the fluid. Similarly, when the system is operating in a cooling mode, a relatively warm heat transfer fluid is circulated through the heat transfer loop to allow heat to be transferred from the fluid to the cooler earth. Thus, the earth can serve as both a heat supplier and a heat sink.
The efficiency of the heat transfer loop is affected by the grout employed to provide a heat exchange pathway and a seal from the surface of the earth down through the hole. The grout needs to have a relatively high thermal conductivity to ensure that heat is readily transferred between the heat transfer fluid and the earth. Further, the grout may form a seal that is substantially impermeable to fluids that could leak into and contaminate ground water penetrated by the hole in which it resides. Even if the fluids do not penetrate the ground water, a seal is still desirable. The hydraulic conductivity, which measures the rate of movement of fluid (i.e., distance/time) through the grout, is thus desirably low. Moreover, the grout needs to have a relatively low viscosity to allow for its placement in the space between the heat transfer loop and the earth, thereby displacing any drilling fluid residing therein. In an attempt to achieve such properties, two types of grouts containing sand to enhance their thermal conductivity, i.e., bentonite-based grout and cement-based grout, have been developed that are extremely labor intensive to prepare. In particular, conventional grouts often require several hundred pounds of sand to render them suitably thermally conductive. Unfortunately, the thermal conductivity that may be achieved by these conventional grouts is limited by the amount of sand that can be incorporated into and properly suspended in the grout. Also, the preparation of such grouts is inflexible in that the concentrations of the components and the mixing procedures must be precise to avoid problems in the field. Further, cement-based grout has the limitation of being very expensive.
A need therefore: exists for an improved grout for use in sealing a heat transfer loop to the earth. It is desirable for the grout to have a higher thermal conductivity and a lower hydraulic conductivity than conventional grouts while at the same time being relatively easy and inexpensive to prepare. It is also desirable for the grout to have some flexibility in the way it can be prepared.
Increasingly, electrical equipment such as high voltage transmission and distribution power lines are being installed (or buried) underground, for safety, ecological, aesthetic, and/or operational reasons. For example, the advantages of buried power lines in tropical regions, where above ground lines are vulnerable to high winds and rains due to tropical storms and hurricanes, are readily apparent However, the capabilities of such installations are limited by the ability of the installations to dissipate heat generated by the flow of electrical power through the equipment. If the thermal resistivity of the environment surrounding the buried equipment is unsatisfactorily high, the heat generated during functioning of the equipment can cause an increase in the temperature of the equipment beyond tolerable limits resulting over time in the premature failure or destruction of the equipment. At the very least, the equipment's life expectancy is decreased, which is an economic disadvantage.
Currently, cable is installed by either digging a trench and backfilling around the cable with a thermally conductive material, or drilling a bore hole, pulling the cable through the bore hole, and placing a thermally conductive material around this cable. The industry typically addresses dissipation of heat around buried power lines in one of two basic ways, both of which involve placing a thermally conductive material around the outside of power line cable (whether or not the cable is strung through a carrier pipe). One way uses bentonite grout to which sand may be added to increase thermal conductivity. The other way uses a cement or similar cementitious material containing sand to provide thermal enhancement. The thermally conductive material is typically installed by either digging a trench and backfilling around the cable with the thermally conductive material or by drilling a bore (hole) and then pulling the cable through the bore containing the thermal enhancement material.
Without sand, bentonite grout does not have high thermal conductivity properties. Typical thermal conductivity values for bentonite grouts range from about 0.4 to about 0.6 BTU/hr-ft-° F. The addition of sand of an appropriate size can increase such thermal conductivity to a range of about 1.0 to about 1.2 BTU/hr-ft-° F. However, the sand can cause placement problems and high pump pressures when positioning as the thermally conductive grout. In horizontal heat loops, high pump pressures can lead to a “frac out” situation where the material induces fractures in the soil through which the material can break through to the surface. Use of cement grout can magnify such problems. Use of sand can also lead to excessive friction, prematurely wearing out pumps and their various parts. For example, in the case of a pipe bundle containing cables, such friction from sand can result in pulling forces that can exceed the strength of the bundle causing the bundle to separate during installation. Backfilling soil with sand added after the pipe installation might be used to avoid such installation friction but backfilling may not always be possible or effective for the fill length of the installation. Further, additional wear caused by the sand to pumps and pump parts remains a concern.